The present invention is generally directed to a variable capacitance indicating circuit and more particularly to a capacitance-to-voltage transformation circuit which provides an output voltage having a magnitude directly proportional to variations in capacitance.
There are many applications where it is desirable, if not necessary, to be able to detect variations in capacitance. For example, in fluid level gauging, two plates are immersed in a fluid and a capacitance between the plates varies as the fluid level between the plates varies. Thus, fluid level may be determined by detecting the value of capacitance between the plates.
As another example, in pressure measuring systems, pressure-to-capacitance transducers have found wide acceptance because they characteristically provide a varying capacitance proportional to pressure variations. As the capacitance of the transducer is detected, the level of pressure may be determined.
These are but two examples of the many enrivonments in which the value of a capacitance is necessary information. In systems designed for detecting the value of a variable capacitance it is desirable to relate the value of capacitance in a readily useful form such as in a voltage having a magnitude directly related to variations in capacitance. The voltage may then be used as a control parameter or for deflecting a suitable meter movement for visually indicating the absolute value of the capacitance, or the absolute or relative variation in capacitance.
In order to provide an output voltage which varies linearly with capacitance changes, many capacitance-to-voltage transformation circuit schemes have been developed. However, the various circuits developed heretofore have all suffered from certain deficiencies which have limited their usefulness.
One variety of capacitance-to-voltage transformation circuit utilizes comparators or Schmitt triggers. Comparators and Schmitt triggers are characteristically sensitive to noise. Thus, such circuits are not suitable for use in environments where there is radiation from relays, ignition noise from automobiles for example or power supply lead noise. The noise generated by these sources would cause faulty operation of the comparators and Schmitt triggers and consequently result in improper detection of the capacitance values.
Another form of capacitance-to-voltage transformation circuit utilizes an oscillator which drives a diode bridge circuit. Such circuits are oscillator-amplitude sensitive and thus are not suitable for use in environments where there is poor supply voltage regulation. Even small variations in supply voltage, and thus small variations in oscillator amplitudes, greatly affects the capacitance value detection of such circuits.
Some variable capacitors, such as the pressure-to-capacitance transducers previously referred to, exhibit a fixed capacitance component and a variable capacitance component. When the value of the variable capacitance is of paramount importance, it is often desirable to cancel the fixed capacitance component to assure proper circuit operation. For example, in some circuit environments if the fixed capacitance component is not eliminated, the output voltage attributable to the fixed capacitance component may be excessively high. As a result, the gain of the capacitance-to-voltage transformation circuit must be reduced so that the circuit components will operate within acceptable and safe voltage limits. With the gain diminished, the ability to detect small changes in variable capacitance is substantially diminished.
It is therefore a general object of the present invention to provide a new and improved capacitance-to-voltage transformation circuit which provides an output voltage having a magnitude which is proportional to variations in capacitance.
It is another object of the present invention to provide a capacitance-to-voltage transformation circuit which is substantially insensitive to extraneous noise.
It is a further object of the present invention to provide a capacitance-to-voltage transformation circuit which is substantially insensitive to oscillator amplitude.
It is a still further object of the present invention to provide a capacitance-to-voltage transformation circuit which cancels the fixed capacitance component of a variable capacitor.